In recent years, optical coherence tomography that forms images of the surface morphology and internal morphology of an object by using a light beam from a laser light source or the like has attracted attention. Unlike an X-ray CT apparatus, optical coherence tomography is noninvasive to human bodies, and is therefore expected to be utilized in the medical field and biological field.
Patent Document 1 discloses a device to which optical coherence tomography is applied. This device has such a configuration that: a measuring arm scans an object by a rotary deflection mirror (a Galvano mirror); a reference arm is provided with a reference mirror; and an interferometer is mounted at the outlet to analyze, by a spectrometer, the intensity of an interference light of light fluxes from the measurement arm and the reference arm. Moreover, the reference arm is configured to gradually change the light flux phase of the reference light by discontinuous values.
The device of Patent Document 1 uses a technique of so-called “Fourier Domain OCT (Optical Coherence Tomography).” That is to say, the device irradiates a low coherence light beam to an object, superposes the reflected light and the reference light to generate an interference light, and acquires the spectral intensity distribution of the interference light to execute Fourier transform, thereby imaging the morphology in the depth direction (the z-direction) of the object. The technique of this type is also called Spectral Domain.
Furthermore, the device described in Patent Document 1 is provided with a Galvano mirror that scans with a light beam (a signal light), and is thereby configured to form an image of a desired measurement target region of the object. Because this device is configured to scan with the light beam only in one direction (the x-direction) orthogonal to the z-direction, an image formed by this device is a two-dimensional tomographic image in the depth direction (the z-direction) along the scanning direction (the x-direction) of the light beam.
Patent Document 2 discloses a technique of scanning with a signal light in the horizontal direction (x-direction) and the vertical direction (y-direction) to form a plurality of two-dimensional tomographic images in the horizontal direction, and acquiring and imaging three-dimensional tomographic information of a measured range based on the tomographic images. As the three-dimensional imaging, for example, a method of arranging and displaying a plurality of tomographic images in the vertical direction (referred to as stack data or the like), and a method of executing a rendering process on a plurality of tomographic images to form a three-dimensional image are considered.
Patent Documents 3 and 4 disclose other types of OCT devices. Patent Document 3 describes an OCT device that images the morphology of an object by sweeping the wavelength of light that is irradiated to an object, acquiring the spectral intensity distribution based on an interference light obtained by superposing the reflected lights of the light of the respective wavelengths on the reference light, and executing Fourier transform. Such an OCT device is called a Swept Source type or the like. The Swept Source type is a kind of the Fourier Domain type.
Further, Patent Document 4 describes an OCT device that irradiates a light having a predetermined beam diameter to an object and analyzes the components of an interference light obtained by superposing the reflected light and the reference light, thereby forming an image of the object in a cross-section orthogonal to the travelling direction of the light. Such an OCT device is called a full-field type, en-face type or the like.
Patent Document 5 discloses a configuration in which the OCT is applied to the ophthalmologic field. Before the OCT device was applied to the ophthalmologic field, a fundus observation apparatus such as a retinal camera had been used (for example, refer to Patent Document 6).
Compared to a retinal camera that can only photograph a fundus from the front, a fundus observation apparatus using OCT has a merit that tomographic images and 3-dimensional images of a fundus are obtained. Therefore, contribution to increase of the diagnosis accuracy and early detection of a lesion are expected.
The fundus observation apparatus using optical coherence tomography thus occupies an important place in diagnosis and treatment of diseases. However, in reality, eyesight values are currently used in order to determine the necessity of treatment and the presence or absence of its effect.
This is due to the fact that the main purpose of treatment is the improvement of eyesight, and the change in the morphology of the fundus (for example, hole shrinkage due to treatment of the macular hole) can be confirmed by the fundus observation apparatus, but it cannot be determined whether or not the change in the morphology results in improvement of eyesight without relying on eyesight measurement.
It should be noted that eyesight measurement is an eye examination in which a visual target for measuring eyesight such as a Landolt ring is presented to the subject, commonly carried out using a subjective optometer (see, for example, Patent Document 7).